1. Field of the Invention
The present invention generally relates to an optical disk signal reproducing apparatus which is designed to irradiate a plurality of laser beams onto an optical disk medium having a high packing recording density and to reproduce information signals recorded therein, and in particular to a crosstalk canceling system for use in the signal reproducing apparatus for canceling crosstalk components of main track information from its adjacent track information by adjusting time differences of individual detected multibeam signals so that the respective reproduced signals have substantially no time lag to accurately cancel the crosstalk components.
2. Description of the Prior Art
In recent years, with the tendency of forming the optical disk medium into high density, a more advanced technique has been required for the optical disk signal reproducing apparatus. Hereinafter, referring to the drawings, explanation is given on an example of the conventional optical disk signal reproducing apparatus. FIG. 7 shows a block diagram of a conventional optical disk signal reproducing apparatus.
In FIG. 7, laser beams L1, L2, and L3 are irradiated respectively on the tracks T1, T2, and T3 formed onto an optical disk medium or carrier 100, and their reflective light beams are projected respectively onto the light receiving elements such as photo detectors 111, 121, and 131, and are outputted as the electric detection signals S1, S2, and S3, respectively. Assuming that the track from which the play-back information signal is desired to be actually received is T2, the laser beams L1, L3 (sub-beams) are for canceling the crosstalk components leaking in from the adjacent tracks T1, T3 while the laser beam L2 (main beam) scans the track T2. In other words, in obtaining the information reproducing signal SX, gain adjusters 113, 133 and addition/subtraction operational amplifier 130 perform the following operation:
SX=S2xe2x88x92kxc3x97(S1+S3)
and the crosstalk components are electrically canceled.
However, owing to the following reasons, the three laser beams to be irradiated on the optical disk medium are required to be positioned at some distance in the tangential direction of the tracks to a certain extent, so that it is not possible to cancel the crosstalk only by the simple addition/subtraction as described above. That is to say, ideally, the laser beams L1, L2, and L3 are required to be irradiated in adjacent relations in the track radial direction. By so practicing, because the track pitch is narrow, the respective laser beams overlap one another, with the inevitable consequence that the beams to be projected onto the light receiving elements mutually overlap, making it impossible to receive the respective beams independently by the light receiving elements 111, 112, and 113. Therefore, it becomes necessary to dispose the respective beams at a certain distance in the tangential direction. At this time, however, there arise time differences in the output signals of the light receiving elements 111, 121, and 131 in proportion to the separation distance in the tangential direction. Accordingly, the time differences that occurs here are corrected by the first and second variable delay elements 112, 122.
In fact, however, it is difficult to control strictly the mutual distances between the laser beams L1, L2, and L3. Even if they could be successfully controlled, it is necessary to set the delay amounts to be generated by the above variable delay elements 112, 122 to be always in optimum condition, because the time differences fluctuate depending on the rotational linear velocity of the optical disk medium. The multiplier 141 detects the correlation between the output signal of the light receiving element 131 and the output signal of the second variable delay element 122, and the multiplier 142 detects the correlation between the output signal of the second variable delay element 122 and the output signal of the first variable delay element 112, respectively. A maximization control means 136 carries out the delay adjustments of the above delay elements 112, 122, respectively, which is disclosed, for example, in the Japanese Patent KOKAI Publication HEI7-176052.
According to the above constitution, however, there are such problems that it is difficult to realize such delay elements 112, 122 that should satisfy both the delay amount and the delay precision, and in the case of using the multipliers 141, 142, the delay amounts cannot be adjusted exactly to the optimum amount because of low detection sensitivity of the correlation signals.
These subjective problems are analyzed as follows. First, the distance between the laser beams L1 and L2 in the track tangential direction or the distance between the laser beams L2 and L3 in the track tangential direction is required to be about 10 xcexcm, in consideration of the configurations and dispositions of the light receiving elements 111, 121, and 131. On the other hand, the recording density of the information recorded in the optical disk medium is, taking an example of a DVD (digital video disk) having PWM recording using, for example, 8-16 modulation codes, about 0.1 xcexcm per 1 channel bit. Accordingly, even the distance of 10 xcexcm corresponds to a difference of as much as 100 bits.
As to the distinctive means for delaying, there has first been considered a constitution to make a series connection of a plurality of analog delay elements in a ladder form. However, the analog delay element itself has a group delay characteristic, and according to a trial calculation, the delay amount in which the group delay does not matter is at most about 10 bits, and it is almost impossible to realize a delay of 100 bits.
As a method for obtaining a delay amount, there can be considered, for example, a sample and hold type filter such as a switched capacitor filter or a digital filter. By using such a filter, it is possible to maintain group delay without respect to the delay amount. However, as the delay precision is determined by the sampling clock in the sample and hold type filter, if it is desired to elevate precision, the sampling clock frequency must be increased, in proportion to which the number of sample holding elements and the operation speed must be increased. In case of using a channel clock for the sampling clock, as 1 clock corresponds to 1 bit delay, if it is desired to make a delay of 100 bits, there are required 100 pieces of sample holding elements, and yet the respective sample holding elements are required to operate at 27 MHz in case of the DVD reproduction at standard speed.
However, in the case of using a channel clock, the delay amount is variable only at the rate of one bit by one bit, and there should be problems in the point of the delay precision. According to an experiment, in order to make idealistic cancellation of crosstalk, matching of the delay of at least xc2xc precision is necessary. When it is desired to realize this state by sample holding, the sampling frequency is required to be 108 MHz or four times the channel clock, and the necessary sample holding elements are increased by a factor of 4 (i.e., 400 elements).
The present invention has been developed to solve these problems and has an object to provide an optical disk signal reproducing apparatus wherein the optimum delay amount can be set very precisely with a simple construction of a lower cost, having a sufficient resolution accuracy, by realizing necessary delay adjustments by using rough delay means of a digital control system constituted by a shift register and fine delay means of an analog control system constituted by group delay filters, and further by executing search of the optimum delay amount by using jitter detecting means.
In order to achieve the object mentioned above, the present invention provides an optical disk signal reproducing apparatus which is provided with an optical disk medium having a plurality of adjacent recording tracks formed thereon to be irradiated with multi laser beams with time differences to generate a plurality of detection signals corresponding to the multi laser beams to thereby obtain a reproduction signal of desired information recorded on a target track while canceling crosstalk components thereof, wherein the apparatus comprises: rough delay means for roughly adjusting a delay amount in delaying preceding detection signals in a rough delay resolution; and fine delay means for finely adjusting a delay amount in delaying the preceding detection signals in a high precision finer than the minimum delay resolution of said rough delay means.
In this construction, the apparatus further comprises delay amount control means for detecting a jitter of the reproduced information signal and controlling the delay amounts of said rough delay means and fine delay means.
According to another aspect of the present invention, there is provided an optical disk signal reproducing apparatus which is provided with an optical disk medium having a plurality of adjacent recording tracks formed thereon to be irradiated with multi laser beams with time differences to generate a plurality of detection signals corresponding to the multi laser beams to thereby obtain a reproduction signal of desired information recorded on a target track while canceling crosstalk components thereof, wherein the apparatus comprises: delay means for delaying preceding detection signals; jitter detecting means for detecting a jitter of the reproduced information signal; and delay amount control means for controlling the delay amounts of said delay means so that the jitter is minimized.
In this construction, said delay means comprises: a rough delay means for providing a roughly adjusting delay amount for delaying the preceding detection signals with a rough delay resolution; and a fine delay means for providing a finely adjusting delay amount for delaying the preceding detection signals with a high precision finer than the rough delay resolution of the rough delay means.
According to another aspect of the present invention, there is provided a crosstalk canceling system for use in an optical disk signal reproducing apparatus, which comprises: a rough delay means for providing a roughly adjusting delay amount for delaying preceding reproduction signals with a rough delay resolution; a fine delay means for providing a finely adjusting delay amount for delaying the preceding reproduction signals with a high precision finer than the rough delay resolution of the rough delay means; a jitter detecting means for detecting a jitter of the reproduction signal; and a delay amount control means for controlling the roughly adjusting delay amount of the rough delay means and the finely adjusting delay amount of the fine delay means so that the jitter is minimized.
By the arrangements as described above, according to the present invention, by providing rough delay means for executing the delay amount in the unit of clock period and fine delay means for continuously generating the delay amount within one clock period, the time difference according to the distance between the main beam and the sub-beam in the track tangential direction can be corrected in high precision. And further, by executing to search the optimum delay amount by using jitter, reproduction signal, optimum delay amount can be set in good precision.